Nanostructured titanium monoboride monolithic material and associated methods

ABSTRACT

A nanostructured monolithic titanium boride (TiB) material and methods of forming such a material are disclosed and described. This material has a room-temperature four-point flexural strength about three times that of commercially available titanium diboride (TiB 2 ). The achievement of nanostructured internal microstructural arrangement having a network of interconnected titanium monoboride whiskers affords a very high strength to this material above some of the best ceramic materials available in the market. The material contains a small amount of titanium, but it is largely made of TiB phase with substantially no TiB 2 . The nanostructured monolithic titanium boride material can be formed by high temperature processing of a powder precursor having carefully selected weight and size distributions of titanium and titanium diboride powders. Potential applications of this material can include wear resistant components such as die inserts for extrusion dies, nozzles, armor, electrodes for metal refining etc. An important advantage of TiB over other hard ceramics is that TiB can be cut by electro-discharge machining (EDM) without difficulty, unlike most ceramics.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application60/680,220, filed May 10, 2005, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

The present invention relates generally to hard ceramic materials suchas metal borides for wear and erosion resistant applications. Moreparticularly, the present invention relates to monolithic nanostructuredtitanium boride material having much higher strength levels compared tomost other hard ceramic materials and other borides.

BACKGROUND OF THE INVENTION

Hard ceramic materials such as oxides, carbides, nitrides and boridesare engineered materials having high melting or decompositiontemperature, high hardness, strength and wear resistance and resistanceto high temperature oxidation and degradation as well as resistance tocorrosive chemicals such as acids and bases. These ceramic materialshave found applications in tools, dies, grinding media, heatingelements, furnace insulations, thermal shields, high temperature probesheathing, and variety of other demanding applications. In particular,the group of compounds known as metal borides are exceptionally hard andchemically inert and are very attractive candidates for high technologyapplications which require high performance as indicated above. Someknown borides are titanium diboride (TiB₂), iron boride (FeB), chromiumboride (CrB), molybdenum boride (MoB), tantalum boride (TaB), zirconiumboride (ZrB₂) and hafnium boride (HfB₂). Of these, only titaniumdiboride is widely manufactured commercially due to its high meltingtemperature, hardness and wear resistance as well as electricalconductivity. Other borides are not widely available due to difficultiesin making pure and dense materials, as well as attendant highmanufacturing costs.

Commercially, titanium diboride is first manufactured as a powder byreacting titanium oxide with boron. The pure titanium diboride powder isthen consolidated by hot pressing above 2200° C. under pressure ofseveral thousand Pascals to produce bulk and solid material. The highertemperatures and pressures involved make this material quite expensive.Hence, use of this material has been limited to specialized andvalue-added markets. Although the high hardness (3200 Kg/mm² Vickershardness) is attractive, hardness exceeding 2000 Kg/mm² is seldomrequired in common applications. Additionally, the high cost of makingtitanium diboride, as well as difficulties in obtaining fully densifiedtitanium diboride limits its flexural strength to about 300 MPa and itsuse to applications where there is no viable alternative.

There have been efforts focused on providing titanium alloys reinforcedwith titanium monoboride whiskers and such materials which have improvedhardness and wear resistance characteristics relative to titanium. Forexample, a number of researchers have formed titanium metal havingtitanium monoboride needles distributed throughout the titanium matrix.Further, some attempts have been made to produce materials having a highcontent of titanium monoboride. However, typically, such materials alsohave substantial amounts of residual titanium metal and titaniumdiboride which significantly reduces the strength of the titaniummonoboride material.

Silicon nitride is among the strongest materials for similar wearresistant applications and can have strength of about 700 MPa. Further,it has a hardness of about 1800 Kg/mm². However, silicon nitride is notelectrically conductive, which means making of complex shapes andprofiles is to be done by diamond-based machining which is complicatedand expensive. Further, titanium diboride which is another commonmaterial for use in wear resistant applications, requires processtemperatures above 2200° C. in order to form the material. Achievingfull density is cumbersome, requiring even higher temperatures,pressures, and process times. Additional problems are more coarse grainsand non-uniform grain structure which are some of the causes ofrelatively low strength (<300 MPa) titanium diboride. The hightemperatures and pressures required significantly increase the costs ofmanufacture.

For this and other reasons, the need remains for methods and materialswhich can provide new or improved materials for use where extremely highstrength alloys are required, which have decreased manufacturing costsand avoid the drawbacks mentioned above.

SUMMARY OF THE INVENTION

It would therefore be advantageous to develop improved materials andmethods which produce a titanium and boron containing monolithicmaterial having significantly increased strength, machinability, andease of manufacture. The present invention provides methods andmaterials which produce nanostructured high strength titanium monoboridemonolithic material which satisfies many of the above criteria.

Titanium diboride (TiB₂) containing one titanium atom and two boronatoms in the chemical structure is a commercially manufactured andwidely used ceramic material. Titanium monoboride is a similar compound,but is made of one boron atom along with one titanium atom in thechemical structure. Titanium monoboride whiskers infiltrating into atitanium mass was previously synthesized by the inventor as a coatingstructured on titanium surfaces as described in International PatentApplication No. WO 2004/046262, which is incorporated herein byreference. However, the present invention provides an avenue forproduction of not previously available or produced monolithic bulktitanium monoboride being substantially free of titanium diboride.Methods for improved synthesis are described in detail along with anevaluation of several useful properties of the titanium monoboride as amonolithic bulk material. Flexural strength evaluations suggest that atroom temperature, the titanium monoboride material is about three timesstronger than TiB₂. This increase in strength arises largely from theextremely fine nanostructure structure of the material and the absenceof substantial amounts of titanium diboride.

The titanium boride (TiB) material of the present invention has aboutthree times the flexural bend strength compared to commerciallyavailable titanium diboride (TiB₂) ceramic material. This is quiteunusual and highly desired in hard material applications for wear anderosion resistance as well as for electrodes in metal refining. Thereare only a few other ceramics with this high level of strength. Inaddition, the titanium monoboride material of the present invention canbe machined by electrical discharge machining which allows foreconomical formation into a wide variety of shapes.

In general, the titanium monoboride of the present invention can behighly useful in any material applications where wear and erosionresistance is required. Examples are dies and inserts in metalprocessing industries (casting and forming etc.), electrodes in aluminumand magnesium extraction, nozzles, armor, and the like.

In one aspect of the present invention, a titanium monoboride articlecan include titanium monoboride whiskers which are present at a volumecontent greater than about 80%. Further, the titanium monoboride articlecan also be substantially free of titanium diboride. The nanostructuredtitanium monoboride articles of the present invention can be readilymachined using electrical discharge machining due to electricalconductivity of titanium monoboride.

The nanostructured monolithic titanium monoboride articles can bemanufactured by forming a powder precursor which includes titaniumpowder and titanium diboride powder. The specific composition andmicrostructure of the powder precursor can govern the process conditionsnecessary to achieve the desired nanostructure. Typically, the powderprecursor can have a titanium powder to titanium diboride powder weightratio from about 0.8:1 to about 1.2:1. The powder precursor can bemaintained under a temperature and pressure sufficient to grow titaniummonoboride whiskers. Temperatures from about 1200° C. to about 1400° C.are typically sufficient to achieve the desired nanostructure. Themonolithic titanium monoboride can then be recovered as a material whichis substantially free of titanium diboride and has titanium monoboridewhiskers present at a volume content greater than about 80%.

Additional features and advantages of the invention will be apparentfrom the following detailed description, which illustrates, by way ofexample, features of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a tri-modal size distribution of powderparticles in the powder precursor in accordance with one embodiment ofthe present invention;

FIG. 2A shows a micrograph of a titanium monoboride material inaccordance with an embodiment of the present invention;

FIG. 2B shows a micrograph of the titanium monoboride material of FIG.2A at a higher magnification;

FIG. 3 is graph of stress versus extension in load displacement resultsof eight different titanium monoboride articles in bending flexuretests;

FIG. 4 is graph of stress versus extension in load displacement resultsof eight different commercial titanium diboride articles in bendingflexure tests; and

FIG. 5 is a graph of cumulative failure probabilities versus fracturestress for titanium monoboride articles of the present invention andseveral commercial titanium diboride samples which highlight thecontrast in increased strength in the titanium monoboride.

It should be noted that the figures are merely exemplary of severalembodiments of the present invention and no limitations on the scope ofthe present invention are intended thereby.

DETAILED DESCRIPTION

Reference will now be made to exemplary embodiments and specificlanguage will be used herein to describe the same. It will neverthelessbe understood that no limitation of the scope of the invention isthereby intended. Alterations and further modifications of the inventivefeatures described herein, and additional applications of the principlesof the invention as described herein, which would occur to one skilledin the relevant art and having possession of this disclosure, are to beconsidered within the scope of the invention. Further, before particularembodiments of the present invention are disclosed and described, it isto be understood that this invention is not limited to the particularprocess and materials disclosed herein as such may vary to some degree.It is also to be understood that the terminology used herein is used forthe purpose of describing particular embodiments only and is notintended to be limiting, as the scope of the present invention will bedefined only by the appended claims and equivalents thereof.

Definitions

In describing and claiming the present invention, the followingterminology will be used.

The singular forms “a,” “an,” and “the” include plural references unlessthe context clearly dictates otherwise. Thus, for example, reference to“a powder precursor” includes reference to one or more of suchmaterials, and “a whisker” includes reference to one or more of suchmaterials.

As used herein, “titanium” without an accompanying element is intendedto refer to elemental titanium in the zero oxidation state. Thus, termssuch as “titanium powder” and “titanium” refer to elemental titanium andspecifically exclude titanium compounds such as titanium diboride,titanium monoboride, etc.

As used herein, “whisker” refers to a nanostructure having a high aspectratio, i.e. the length to diameter ratio greater than about 5:1.Typically, whiskers have a generally polygonal cross-section; howevercross-sections may vary somewhat, e.g., hexagonal, diamond, andcircular. Whisker diameters are most frequently in the nanometer range;however diameters can vary from about 50 nm to about 3 μm, althoughpreferred diameters are from about 100 nm to about 600 nm.

As used herein, “nanostructure” is intended to indicate that at leastone physical dimension of the material is in the nanometer range, i.e.less than 1 μm, and preferably less than about 800 nm.

As used herein, “packing factor” refers to the ratio of volume occupiedby solids to volume of a unit cell. Thus, a packing factor for mixturesof particles is independent of absolute size and is directly related torelative sizes.

As used herein, “substantial” when used in reference to a quantity oramount of a material, or a specific characteristic thereof, refers to anamount that is sufficient to provide an effect that the material orcharacteristic was intended to provide. The exact degree of deviationallowable may in some cases depend on the specific context. Similarly,“substantially free of” or the like refers to the lack of an identifiedelement or agent in a composition. Particularly, elements that areidentified as being “substantially free of” are either completely absentfrom the composition, or are included only in amounts which are smallenough so as to have no measurable effect on the composition.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary.

Concentrations, dimensions, amounts, and other numerical data may bepresented herein in a range format. It is to be understood that suchrange format is used merely for convenience and brevity and should beinterpreted flexibly to include not only the numerical values explicitlyrecited as the limits of the range, but also to include all theindividual numerical values or sub-ranges encompassed within that rangeas if each numerical value and sub-range is explicitly recited. Forexample, a size range of about 1 μm to about 200 μm should beinterpreted to include not only the explicitly recited limits of 1 μmand about 200 μm, but also to include individual sizes such as 2 μm, 3μm, 4 μm, and sub-ranges such as 10 μm to 50 μm, 20 μm to 100 μm, etc.

Titanium Monoboride Articles

In accordance with the present invention, a titanium monoboride articlecan include a densely packed mass of titanium monoboride whiskers as abulk material. The titanium monoboride whiskers can be present at avolume content greater than about 80% such that the primary constituentof the article is titanium monoboride whiskers which are intergrown andform an interconnected network of nanostructure whiskers. Further, thetitanium monoboride articles of the present invention can besubstantially free of titanium diboride. The substantial elimination oftitanium diboride from the final product can be achieved using themethods described in more detail below. In one aspect, the titaniummonoboride articles of the present invention can consist essentially ofthe titanium monoboride whiskers and titanium. In a detailed aspect, thetitanium monoboride article can be completely free of titanium diboride.

The nanostructure of the present invention is one important aspect whichdetermines the beneficial extraordinary improvements in strength andmechanical properties. The volume content of titanium monoboridewhiskers and the dimensions of such whiskers typically should fallwithin certain ranges in order to achieve the desired results. Forexample, although many titanium monoboride articles of the presentinvention containing greater than about 80% by volume of whiskers can bebeneficial, the titanium monoboride articles are characterized by a hightitanium monoboride whisker content. Typically, the titanium monoboridewhisker volume content can be from about 85% to about 95%, and ispreferably from about 88% to about 93%. In addition, the titaniummonoboride whiskers can have a nanostructure wherein the averagediameter of the titanium monoboride whiskers is from about 10 nm toabout 900 nm, and preferably from about 20 nm to about 200 nm. It isthought that the whisker nanostructure of these articles is largelyresponsible for the increase in mechanical strength. Typically, thetitanium monoboride whiskers can have an average length of from about 10μm to about 700 μm, and frequently from about 50 μm to about 300 μm. Thelength of the whiskers is sufficient to allow most of the titaniummonoboride whiskers to form an interconnected network. In an additionalrelated measure of the nanostructure, the titanium monoboride whiskerscan have an average aspect ratio from about 5:1 to about 500:1, althoughother aspect ratios can be suitable.

In yet another aspect of the present invention, the titanium monoboridearticles are electrically conductive. This electrical conductivity isrelatively unique among ceramics. For example, ceramics such as siliconnitride or silicon carbide are not sufficiently conductive to allowmachining via electrical discharge machining (EDM). Thus, such materialsare usually machined using relatively expensive superabrasive tools suchas diamond tools. In contrast, the nanostructured titanium monoboridearticles of the present invention can be readily machined using EDM suchthat highly complex or simple shapes can be formed without the need forexpensive diamond tools.

Exemplary Manufacturing Methods

The specific process and conditions for formation of the titaniummonoboride articles of the present invention can be carefully chosen andimplemented in order to achieve the nanostructured titanium monoboride.In one aspect, a method of forming a monolithic titanium monoboridehaving a whisker nanostructure can include forming a powder precursor.Suitable powder precursors include titanium powder and titanium diboridepowder. In order to achieve the desired nanostructure and absence oftitanium diboride, the relative powder sizes and weight ratio areimportant. In addition, the powder precursor can have a titanium powderto titanium diboride powder weight ratio from about 0.8:1 to about1.2:1. Weight ratios from about 0.9:1 to about 1.1:1 can also be used.However, currently preferred formulations of the powder precursorinclude a greater amount of titanium diboride by weight such as about0.95:1 to about 0.99:1, and most preferably about 0.96:1. A ratio ofabout 0.96:1 corresponds to a weight ratio of 49:51 when the powderprecursor consists essentially of titanium powder and titanium diboridepowder. Typically, weight ratios within about +/−2 wt % of the 49:51ratio are preferred. These ratios can be adjusted to account for anysubstantial presence of other elements such as Zr, O, N, C, Fe, and thelike.

As mentioned herein, the particle sizes of the respective powders canalso govern the ability of the powder precursor to form nanostructuredtitanium monoboride, as well as can affect the operating temperature andprocess time. Further, careful selection of relative powder sizes canhelp to achieve more uniform whisker growth and allow the extent ofreaction to be driven to completion at the lowest possible processtemperature. However, as a general guideline, the titanium powder canhave a particle size from about 20 μm to about 100 μm, and preferablyabout 45 μm. Similarly, the titanium diboride powder can have a particlesize from about 1 μm to about 4 μm, and preferably about 2 μm. Thus, thetitanium powder can preferably have a particle size significantlygreater than the particle size of the titanium diboride. Typically, thetitanium powder can have a particle size from about 5 to about 100 timesthat of the titanium diboride, and most often from about 8 to about 15times.

In one currently preferred aspect of the present invention, the powderprecursor can have a tri-modal size distribution. Particularly, thetitanium powder can include titanium powders having at least twodifferent sizes. In one aspect, the titanium powder can have twodifferent sizes. Although additional quantities of titanium powderand/or titanium diboride powder at different sizes can be added to thetitanium powder, only minor improvement in results is achieved. Onetri-modal size distribution that has proven particularly effectiveincludes a first quantity of titanium powder, a second quantity oftitanium powder, and a titanium diboride powder in a size ratio ofX:Y:Z, respectively, where X is from about 35 to about 55, Y is fromabout 2 to about 15, and Z is from about 1 to about 5. Preferably, X canbe from about 40 to about 50 and Y from about 5 to about 10, and mostpreferably a ratio of 45:7:2 can be used.

As mentioned above, providing a tri-modal size distribution is onefactor which allows the production of nanostructured titanium monoboridemonolithic material in accordance with the present invention. Althoughthe invention is not specifically limited thereto, it has been foundvery difficult to achieve uniform growth and substantial elimination oftitanium diboride using bi-modal size distributions (i.e. titaniumpowder and titanium diboride powder each of a single average size). Atleast part of the reason for this is the influence of packing density onthe ability of titanium and boron to migrate and react. Thus, theformation of the powder precursor is a careful balance of packingdensity and access of boron to titanium to form titanium monoboridewhiskers. Access of boron to titanium is at least partially governed bythe relative proportions of titanium and titanium diboride, as well asthe relative particle sizes.

In tri-modal mixtures of solid state powders, the packing density is astrong function of the particle size ratios of the three powders. Aproper selection of the powder sizes, to achieve maximum packing beforesintering, is important in achieving high density, complete reaction,and uniform distribution of the nanostructure in the TiB monolithicmaterial. The mathematical formula, given in R. M. German, PowderPacking Characteristics, Published by Metal Powder IndustriesFederation, Princeton, N.J., 1994, p. 183, which is incorporated hereinby reference, for powder-packing density (f) for a generic tri-modalpowder, is given in Equation 1. $\begin{matrix}{{f = {0.951 - {0.098A_{1}} + {0.098A_{2}} - {0.198A_{1}A_{2}}}}{where}{{A_{1} = {\exp\left( {{- 0.201}\frac{D_{L}}{D_{M}}} \right)}},{A_{2} = {\exp\left( {{- 0.201}\frac{D_{M}}{D_{S}}} \right)}},}} & (1)\end{matrix}$and D_(S), D_(M) and D_(L) are the diameters of the small, medium andthe large particles, respectively. According to Equation 1, fortri-modal powder mixtures, the maximum possible packing densityattainable in the present tri-modal packing is about 92-93%, which isquite high compared to conventional powder processing in industrialoperations that use mostly mono-sized powders. For example, oneembodiment of the tri-modal mixtures used in the present invention has aparticle size ratio of 45:7:2 (large to medium to small) which is veryclose to the theoretical maximum size ratio of 49:7:1, for obtaining amaximum packing density. It is therefore clear that the tri-modallypacked powders had an initial packing density that was quite close tothe ideal packing density. The maximization of powder packing density isone important factor in achieving a full densification, completereaction, absence of remnant unreacted TiB₂ particles, and uniformdistribution of the nanostructure in the present TiB monolithicmaterial.

FIG. 1 illustrates various aspects of particle packing and theparticle-surrounding-mechanism that can be considered in choosing thetri-modal powder packing to manufacture the present nanostructuredmonolithic material. When the present tri-modal powders are blendedthoroughly, both the large (e.g. 45 micron) as well as small size (e.g.7 micron) titanium powders will be completely surrounded by smallertitanium diboride particles. This spatial arrangement enables uniformcontact between titanium and titanium diboride particles, allowinguniform reaction and formation of TiB whiskers. This distribution ofparticles is also important to have complete reaction and prevent anyresidual TiB₂ particles. Without being bound to any particular theory,it is thought that the medium size titanium particles will act as thegenesis of whisker growth and the larger titanium and smaller titaniumdiboride particles will act dominantly as raw material sources fromwhich material diffuses toward growth areas. Additionally, the mediumsize titanium particles allow for increased packing density above thatpossible using a bi-modal size distribution. In one aspect of thepresent invention, the tri-modal size distribution can be determined byoptimizing packing density using Equation 1 to within about 5% of thetheoretical maximum packing density.

The titanium powder and titanium diboride powders can then be blended toobtain a substantially homogeneous powder mixture. This can beaccomplished by mixing using a high shear mixer, ball mill or rotaryblender with steel balls, or the like. The powder mixture can be pressedor consolidated to form the powder precursor to reduce porosity.Although the specific powder precursor can vary in properties,typically, the precursor can have a packing density from about 88% toabout 95%, and preferably about 90%.

Almost any suitable commercial source can be used to obtain the abovepowders. Alternatively, the powders can be formed using any number ofpowder synthesis processes. However, regardless of the commercial sourceof such powders, the powders typically include nominal amounts ofimpurities such as O, N, C, Fe, Zr, H, and the like. Some of theseimpurities can be removed or reduced through flushing with argon gasunder vacuum or other heat treatments. Typically, such impurities in anamount less than about 1% total (preferably less than about 0.8% total)impurities does not significantly affect ultimate performance of thenanostructured titanium monoboride articles.

The powder precursor can then be maintained under a temperature andpressure sufficient to grow titanium monoboride whiskers. As a generalguideline, the specific process temperature strongly affects the shapeand diameter of the whiskers. In accordance with the present invention,the process temperature can be from about 1200° C. to about 1400° C.,and preferably from about 1250° C. to about 1350° C. In one aspect wherethe titanium powder is about 45 μm and the titanium diboride powder isabout 2 μm, the process temperature can be about 1300° C. Similarly, thepressure can range from about 26 MPa to about 30 MPa, and preferablyabout 28 MPa. In an alternative aspect, the tri-modal packing of powderscan allow a process temperature down to about 1100° C.

The powder precursor can be heated under pressure and maintained thereatfor a time which is sufficient to form titanium monoboride whiskershaving the desired nanostructure. Similarly, process time can influencewhisker length and thus the thickness of the surface hardened region.Increasing process times can also result in a thickening of thewhiskers. Typically, the process time can range from about one and ahalf to about three hours, and preferably about two hours. However, thisprocess time can vary depending on the particle size and pressure.

Additionally, the powdered precursor can be heated to the appropriateprocess temperature at a rate of about 30° C./minute. Those skilled inthe art will recognize, however, that this is merely a guideline andthat temperatures and times outside those indicated may also be used toachieve the desired nanostructure.

As an additional consideration in designing the process to achieve thedesired nanostructure, the particle size of each constituent can affectthe process temperature and process time needed to achieve the desirednanostructure and composition. Incorrect performance and/ordetermination of the process temperature or time based on a given powderprecursor can result in unacceptable products. For example, processtimes in excess of the appropriate time can result in the titaniummonoboride whiskers growing together to form solid portions such thatthe whisker nanostructure is lost. Conversely, excessively short processtimes can leave the whiskers insufficiently interconnected and furtherallow residual titanium diboride to remain in the matrix. Aninappropriate temperature can also prevent the significant formation ofhigh volume contents of titanium monoboride whiskers. The necessaryprocess time and temperatures can be determined based on calculationswhich take these variable into account.

On the basis of diffusion data, the time needed to form TiB whiskerphases by the reaction between titanium and TiB₂ powders can bedetermined. The growth of TiB phase follows a parabolic relationshipdescribed in Equation 2.x=k√{square root over (t)}  (2)where x is the length of the whiskers, k is the growth rate, and t isthe process time. The temperature dependence of TiB whisker growth ratecan be expressed as using Equation 3. $\begin{matrix}{k = {k_{0}{\exp\left( {- \frac{Q_{k}}{2{RT}}} \right)}}} & (3)\end{matrix}$where k₀ is a constant (experimentally determined frequency factor),Q_(k) is the activation energy for growth, T is the temperature, and Ris the universal gas constant (i.e. 82.05 cm³ atm/K/mol). The values ofk₀ and Q_(k) were found to be 17.07×10⁻⁴ m/√sec and 190.3 kJ/mole in Z.Fan, Z. X. Guo, and B. Cantor: Composites, 1997, vol. 28A, pp. 131-140,which is incorporated herein by reference. From these values, thecomputed k values at 1300° C. and 1100° C. are 40.96×10⁻⁸ m/√sec and118.2×10⁻⁸ m/√sec, respectively. From these data, the estimated lengthsof the TiB whiskers that can form after 2 hrs, assuming direct Ti—TiB₂contact, are about 99 μm at 1300° C. and 34 μm at 1100° C. However,experiments reveal that the growth of TiB substantially ceases afterreaching length values of about 40-45 microns and 10-15 microns for thepowder mixtures pressed at 1300° C. and 1100° C., respectively. Thesevalues are roughly half of the estimated values. This suggests that TiBgrowth is fully complete prior to the theoretical two hour process timeand that the present TiB material can be formed in about a one hourprocess time, to get nearly the same microstructures. However, shortertimes can result in remnant TiB₂ which will disrupt the uniformity ofthe nanostructure; therefore, preferably, process times of up to abouttwo hours can be used in order to ensure that all TiB₂ will be fullyconverted into TiB. As a general guideline, process times from abouthalf the theoretical process time to about 1.2 times the theoreticalprocess time are preferred.

Generally, upon heating the powder precursor titanium powder reacts withtitanium diboride in an oxidation-reduction reaction to form a titaniummonoboride as indicated by Equation 4.Ti+TiB₂→2TiB  (4)

The pressure and temperature can then be reduced to allow the titaniummonoboride article to cool. A monolithic titanium monoboride can thus berecovered which is substantially free of titanium diboride. Further, thetitanium monoboride whiskers can be present at a volume content greaterthan about 80% in accordance with the principles of the presentinvention.

Depending on the powder precursor composition and the process conditionsused in forming the titanium monoboride articles of the presentinvention, the strength can generally range from about 600 MPa to about950 MPa, and preferably from about 650 MPa to about 900 MPa. Thehardness of the titanium monoboride can range from 1600 Kg/mm² to 1800Kg/mm², measured by the Vickers technique.

Several non-limiting examples of suitable applications which canincorporate the nanostructured titanium monoboride monolithic materialof the present invention can include, but is in no way limited thereto,bearings, tribological surfaces, grinding media, armor, dies, insets,heating elements, crucibles, nozzles, electrodes, and the like.

The following examples illustrate exemplary embodiments of theinvention. However, it is to be understood that the following is onlyexemplary or illustrative of the application of the principles of thepresent invention. Numerous modifications and alternative compositions,methods, and systems may be devised by those skilled in the art withoutdeparting from the spirit and scope of the present invention. Theappended claims are intended to cover such modifications andarrangements. Thus, while the present invention has been described abovewith particularity, the following example provides further detail inconnection with what is presently deemed to be a practical embodiment ofthe invention.

EXAMPLES Example 1

Preparation

A nanostructured titanium monoboride was manufactured by carefullycontrolled reaction sintering of a tri-modal distribution of Ti and TiB₂powders. The relative powder sizes were important in obtaining thedesired nanostructure in the final TiB material. A bi-modal titaniumpowder mixture having an average size of 45 μm and 7 μm and acomposition of (wt. %) 0.23 O, 0.02 N, 0.01 C, 0.04 Fe, and 0.024H andtitanium diboride powders having an average size of 2 μm and acomposition of (wt. %) 30.3 B, 0.67 Zr, 0.01 C, 0.04 Fe, and 0.024H wereprovided. The titanium powder and titanium diboride powders were mixedtogether at 49 wt % Ti (41±2 wt % of 45 μm size and 9±1 wt % of 7 μmsize powder) and 51 wt % TiB₂. The powders were then thoroughly blendedfor 24 hours in a rotary blender with steel balls to ensure homogeneityof the mixture. These powders have a size ratio of 45:7:2, and a packingdensity of about 90% in the blended state, which was found to beimportant in achieving the desired nanostructure.

The reaction sintering process can be performed in a single heating andpressure step. The blended and packed powder mixture was placed in a 30ton vacuum hot press in a graphite die. The blended powders were loadedinto the die with GRAFOIL sheets as liners along the die walls. The hotpress chamber was then evacuated using a rotary pump and refilled withcommercially pure argon. The evacuation and refilling procedure wasrepeated three times to remove any residual O₂ and N₂ in the chamber.The die assembly with the powder was then heated at a rate of 30° C./minto 1300° C., which is the temperature at which the blended Ti and TiB₂powders react to form the TiB whiskers. Upon reaching 1300° C., apressure of about 28 MPa was applied and held for two hours at thistemperature. Small changes in temperature (e.g. +/−50° C.) and pressure(+/−2 MPa) are not likely to change the final result greatly, althoughcare should be taken to eliminate residual titanium diboride. The diewas subsequently cooled slowly to room temperature and a dense,monolithic TiB material was ejected out of the die. The material wasready for preparation into desired components without any furthertreatment or processing. This material is particularly suited tomachining via electro-discharge machining (EDM) processes.

Evaluation and Testing

FIGS. 2A and 2B illustrate the nanostructures of the titanium monoboridematerial. The micrographs were taken in a scanning electron microscopeafter polishing using diamond compounds/disks and deep etching withKroll's reagent. The microstructure consists of bundles of extremelythin-sized whiskers. The individual rod-like features are thenanostructured TiB whiskers. The nanostructured nature of this TiBarticle is clearly evident in the micrographs, although the most of theindividual rods are not entirely distinguishable due to the limitationsof resolution in the scanning electron microscope in which these imageswere taken. The volume fraction of titanium monoboride, estimated fromX-ray diffraction varied from 86-95%, with the reminder being Ti. Thedark regions in the micrographs are believed to be made of the residualtitanium metal, which was removed by the etching process, which was doneto reveal the TiB whiskers.

The long needle-like features are the TiB whiskers that formed as aresult of the reaction between Ti and TiB₂ particles in the powderprecursor mixture. The darker regions represent the residual titanium(anywhere between 5-9%, estimated from X-ray diffraction, depending onlocation) that acts as a binder of the TiB whiskers.

Four-point flexural strength tests were conducted in an Instronmechanical testing machine. The size of the samples was approximately 25mm×6 mm×5 mm. The samples were cut from the reaction sintered plate byEDM machining. Subsequently, the samples were ground using 220 gritdiamond wheel to remove surface layers about 0.25 mm depth from the EDMsurface. The samples were polished further using a 35 micron diamonddisk, followed by fine polishing through 10, 6 and 3 micron suspensionssequentially. The flexure strength tests were conducted in four-pointbending at a displacement loading rate of 0.2 mm/sec. For comparison,specimens from commercial TiB₂ plate were also cut, prepared and testedin an identical manner.

FIG. 3 presents the stress-displacement plots of tests of severalspecimens from the nanostructured titanium monoboride material. FIG. 4presents the same test results for the commercial TiB₂ material. It canbe seen that the ultimate failure stresses for the specimens ofnanostructured TiB are about two to three times higher than thecorresponding loads for the TiB₂ specimens.

The flexural strength levels for both materials are plotted in the formof a cumulative failure probability distribution in FIG. 5. Based onthese results, the flexure strength levels of nanostructured TiBmaterial is, on average, about three times higher than the commercialTiB₂ material. This is somewhat unexpected due to fact that bulktitanium diboride, TiB₂ is a harder material than bulk titaniummonoboride, TiB. However, the nanostructure refinement achieved in thetitanium monoboride articles of the present invention, yielding awhiskered nanostructure clearly makes the material much stronger thanTiB₂. The grain size of TiB₂ used for comparison in FIG. 5, is in therange of 5-10 microns.

It is to be understood that the above-referenced arrangements areillustrative of the application for the principles of the presentinvention. Thus, while the present invention has been described above inconnection with the exemplary embodiments of the invention, it will beapparent to those of ordinary skill in the art that numerousmodifications and alternative arrangements can be made without departingfrom the principles and concepts of the invention as set forth in theclaims.

1. A titanium monoboride article, comprising titanium monoboridewhiskers, said titanium monoboride whiskers being present at a volumecontent greater than about 80% and said article being substantially freeof titanium diboride.
 2. The article of claim 1, wherein the articleconsists essentially of the titanium monoboride whiskers and titanium.3. The article of claim 1, wherein the article is free of titaniumdiboride.
 4. The article of claim 1, wherein the titanium monoboridewhisker volume content is from about 85% to about 95%.
 5. The article ofclaim 1, wherein said titanium monoboride whiskers have a nanostructurewherein the average length of the titanium monoboride whiskers is fromabout 5 nm to about 100 μm.
 6. The article of claim 1, wherein thetitanium monoboride article is electrically conductive.
 7. The articleof claim 1, wherein the titanium monoboride article has a strength fromabout 650 MPa to about 950 MPa.
 8. A method of forming a monolithictitanium monoboride having a whisker nanostructure, comprising the stepsof: a) forming a powder precursor including titanium powder and titaniumdiboride powder, said powder precursor having a titanium powder totitanium diboride powder weight ratio from about 0.8:1 to about 1.2:1;b) maintaining the powder precursor under a temperature and pressuresufficient to grow titanium monoboride whiskers, said temperature beingabout 1200° C. to about 1400° C.; and c) recovering the monolithictitanium monoboride being substantially free of titanium diboride, saidtitanium monoboride whiskers being present at a volume content greaterthan about 80%.
 9. The method of claim 8, wherein the weight ratio isfrom about 0.9:1 to about 1.1:1.
 10. The method of claim 9, wherein theweight ratio is about 0.96:1.
 11. The method of claim 8, wherein thetitanium powder has a particle size from about 20 μm to about 100 μm andthe titanium diboride powder has a particle size from about 1 μm toabout 4 μm.
 12. The method of claim 8, wherein the titanium powder has asize ratio of titanium powder particle size to titanium diborideparticles size from about 15:1 to about 30:1.
 13. The method of claim 8,wherein the powder precursor has a tri-modal size distribution such thatthe titanium powder includes a first quantity of titanium powder havinga first average size and a second quantity of titanium powder having asecond average size.
 14. The method of claim 13, wherein the tri-modalsize distribution is formed of the first quantity of titanium powder,the second quantity of titanium powder, and the titanium diboride powderin a size ratio of X:Y:Z, respectively, where X is from about 35 toabout 55, Y is from about 2 to about 15, and Z is from about 1 to about5.
 15. The method of claim 13, wherein the tri-modal size distributionis determined by optimizing packing density to within about 5% of atheoretical maximum.
 16. The method of claim 8, wherein the temperatureis about 1250° C. to about 1350° C.
 17. The method of claim 8, whereinthe step of forming includes blending the titanium powder and thetitanium monoboride powder to obtain a substantially homogeneous powdermixture.
 18. The method of claim 8, wherein the powder precursor has apacking density from about 88% to about 95%.
 19. The method of claim 8,wherein the pressure is from about 26 MPa to about 30 MPa.
 20. Themethod of claim 8, wherein the step of maintaining is performed for aprocess time from about half a theoretical time to about 1.2 times thetheoretical time, said theoretical time, t, being calculated from thefollowing two equations:$x = {{k\sqrt{t}\quad{and}\quad k} = {k_{o}{\exp\left( \frac{- Q_{k}}{2{RT}} \right)}}}$where x is titanium monoboride whisker length, k_(o) is frequencyfactor, Q_(k) is activation energy of whisker growth, T is thetemperature, and R is the universal gas constant.
 21. The method ofclaim 8, wherein the step of maintaining is performed for a time of fromabout one and a half to about three hours.
 22. The method of claim 8,further comprising the step of heating the powdered precursor to thetemperature at a rate of about 30° C./minute prior to the step ofmaintaining.
 23. The method of claim 8, further, comprising the step ofelectro-discharge machining the monolithic titanium monoboridesubsequent to the step of recovering.